1. Field of the Invention
It is well known that oil bearing rock characteristics such as porosity can be monitored by making use of nuclear magnetic resonance (NMR) techniques. Conventionally, test samples of rock cores from the well head are extracted and then sent to a remote laboratory for analysis. However, this is time consuming and there is a risk of damage to the rock core before it is analysed. Consequently, it has been proposed to carry out NMR analysis on rock cores at the well site. An example of apparatus of this type is described in U.S. Pat. No. 4,885,540. In this case, rock core samples are placed in individual troughs and fed by conveying rollers between the poles of an electromagnet and through a rf coil. The coil is pulsed in a conventional manner and used as a receiving coil to perform various conventional NMR analyses on the rock core samples.
2. Description of the Related Art
A number of problems arise with this known approach. One particular problem is that large fringe fields are generated by the magnet which is particularly undesirable when the apparatus is being used at the well head site. In this situation, there will tend to be large bodies of iron close to the equipment which experience the fringe field and as a result significantly reduce the uniformity of the field in the working region while there will also be sensitive instrumentation nearby which is affected by the fringe field.
Another problem which arises with the conventional approach is that the NMR analysis is carried out on the full cross-section of the rock cores which can lead to erroneous results due to the likelihood of damage to the surface areas.